Internally-calibrated, two-detector gas filter correlation radiometry (GFCR) system

ABSTRACT

An entrance aperture of a GFCR system receives light from a scene of interest. The light is focused to form an image at a focal plane. Light associated with a selected field-of-view of the image at the focal plane is then confined to a spectral band at which a gas of interest absorbs. The confined light is split into first and second paths. A calibrating light is selectively produced from within the optical train at the focal plane. A portion of the calibrating light traverses each of the first and second paths. A region that is substantially non-interfering with respect to the spectral band is disposed in the first path. A gas cell filled with the gas of interest is disposed in the second path. The light passed through the region and through the gas cell is independently detected and used to generate output signals indicative of the light so-detected. The output signals are processed with the output signals generated by the light from the selected field-of-view of the image being used to generate a measure of the gas of interest and the output signals generated by the portion of the calibrating light traversing the first and second paths being used to calibrate the GFCR system.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is co-pending with one related patentapplication entitled “TWO-DETECTOR GAS FILTER CORRELATION RADIOMETRY(GFCR) SYSTEM USING TWO-DIMENSIONAL ARRAY DETECTION OF DEFOCUSED IMAGEAND DETECTED-SIGNAL SUMMATION”, filed by the same inventor and owned bythe same assignee as this patent application.

FIELD OF THE INVENTION

The invention relates generally to gas filter correlation radiometry(GFCR), and more particularly to a two-detector GFCR system thatincludes a controllable internal light-producing source for use incalibrating the GFCR system.

BACKGROUND OF THE INVENTION

“Gas filter correlation radiometry” (GFCR) is an optical remote sensingmethod used to produce highly sensitive measurements of “targeted”gases. A conventional GFCR measurement system using two single-elementdetectors is shown in FIG. 1 and is referenced generally by numeral 10.The basic elements of GFCR system 10 form an optical train thatincludes:

focusing optics (e.g., a telescope) 12 that focuses the image containedin light 200 onto a field stop 14 that sets the field-of-view of thelight focused by optics 12,

optics 16 that collimate the light passing through field stop 14,

a chopper 18 used to modulate the collimated light,

a spectral filter 20 that confines the light (collimated by optics 16)to a specific spectral bandpass where (spectrally) a gas of interestabsorbs,

optics 22 for splitting the spectrally filtered light into two paths 24and 26 where path 24 defines a region that is non-absorbing within thespectral bandpass at which the gas of interest absorbs,

a single-element, light-intensity detector 28 disposed in path 24, and

a gas cell 30 filled with a gas of interest (i.e., the target gas) anddisposed in path 26 such that the light traveling therealong passesthrough gas cell 30 prior to impinging on a single-element,light-intensity detector 32.

GFCR system 10 further uses “back-end” electrical components thatincludes balancing electronics 40 coupled to the outputs of detectors 28and 32. In general, balancing electronics 40 include a balancingamplifier 42, a differential amplifier 44 and a gain amplifier 46 thatcooperate to measure a difference between the outputs of detectors 28and 32.

GFCR system 10 uses a sample of the gas to be detected (i.e., the targetgas) as a filter for removing sensitivity to that gas in path 26. Thatis, the light is passed through field stop 14, collimated, andspectrally filtered with broadband filter 20 to confine the light to aspectral bandpass where the target gas absorbs. After the beam is splitby optics 22, gas cell 30 absorbs light from spectral wavelengthscoinciding with spectral absorption features (i.e., typically spectralabsorption lines) of the target gas.

In practice, the detector signals from detectors 28 and 32 areelectronically balanced to be approximately equal when viewing light 200from an unattenuated light source such as the sun observed above theatmosphere from a satellite. Then, when the light source is observedthrough the atmosphere during solar occultation, a difference betweenthe two signals is induced and measured. Absorption by the target gas inthe observed scene attenuates the vacuum path signal generated bydetector 28. However, the gas path signal generated by detector 32 isminimally attenuated. The difference signal (i.e., the differencebetween the two signals generated by detectors 28 and 32) is highlysensitive to and correlated with the amount of target gas in theline-of-sight of GFCR system 10.

Useful GFCR measurements must be tailored to the absorptioncharacteristics of the target gas, and depend on the ability to maintaina stable and calibrated gas cell containing a sample of the target gas.For example, when the GFCR method was employed in a solar occultationexperiment (i.e., the “halogen occultation experiment” or HALOE),sensitivities of 10⁻⁵ in mean band absorption were achieved by a systemsimilar to that depicted in FIG. 1. The two detectors' signals weredifferenced and balanced by electronics 40 to give nearly zerodifference during solar observation above the atmosphere. The differencesignals measured during solar occultation were used very successfully asmeasures of target gas absorption. To achieve high precision, thedifference signals included an additional gain of one hundred or more.The key to making these measurements is the ability to determine thebalance and rate of change of the difference signal immediately beforethe observation, which mitigates error due to drifts in detectorresponse. To achieve the desired measurement accuracy of 1 part in 10⁵,the balance must be known to 10⁻⁵ of the full broadband signal. Thus,small drifts in detector response, if not detected and corrected, canseverely corrupt the difference measurement.

In addition to use in solar occultation, there has been hope that GFCRcould be used for solar backscatter measurements, which could yield muchbetter geographical coverage than solar occultation and be realizedusing small commercial devices. However, because the conventionaltwo-detector method requires continuous high-precision calibration ofthe balance condition (i.e. calibration of the signal drift due tochanges in system response), most researchers have abandoned thetwo-detector method in favor of single-detector methods. Unfortunately,while single-detector methods can nearly eliminate detector instabilityas an error source by measuring both signals with the same detector,they introduce a host of other problems, depending on method ofimplementation. For example, if the gas cell condition is modulated bychanging pressure or optical mass, there is a significant decrease insensitivity because the cell modulation produces a relatively smallspectral difference between paths. The signals are also difficult tomodel because of gas heating and cell state variation that may not reachuniform equilibrium.

In another single-detector method, the light path is switched between agas-cell path and a non-gas-cell (e.g., vacuum) path by either rapidlyre-routing the beam (e.g., polarization switching techniques) or movingthe gas cell into and out of the beam. However, both of these approachesintroduce noise due to beam steering and loss of signal integration timedue to time between modulated states.

An even greater problem with any single-detector method is the loss ofmeasurement simultaneity and/or the ability to exactly matchfield-of-views for gas and vacuum paths. If the scene changes during thetime necessary to switch between modulated states or because offield-of-view mismatch, the change in normalized difference signal(caused by scene brightness variation) will corrupt the datainterpretation that assumes the difference signal is produced solely byspectral variation. For example, a satellite traveling at 7 km/secencountering a 1% per kilometer change in mean scattering brightnessover the field-of-view will experience a fractional brightness change of10⁻⁴ in 1.4 milliseconds which would be falsely interpreted as spectralvariation. This presents a severe problem for the single-detectormethod, or any method that does not make simultaneous and spatiallyidentical measurements of the two states (i.e., gas path and vacuumpath).

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a GFCRsystem.

Another object of the present invention is to provide a GFCR system thatemploys a two-detector methodology.

Still another object of the present invention is to provide a GFCRsystem that can be accurately calibrated.

Yet another object of the present invention is to provide a GFCR systemthat is easily calibrated.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a gas filter correlationradiometry (GFCR) system includes an optical train having an entranceaperture at which light from a scene of interest is received. The lightis focused to form an image at a focal plane within the optical train.The optical train confines light from a selected field-of-view of theimage at the focal plane to a spectral band at which a gas of interestabsorbs. The optical train splits the confined light into first andsecond paths. A calibrating light is selectively produced from withinthe optical train at the focal plane. A portion of the calibrating lighttraverses each of the first and second paths. A region that issubstantially non-interfering with respect to the spectral band isdisposed in the first path. A gas cell filled with the gas of interestis disposed in the second path. The light passed through the region andthrough the gas cell is independently detected and used to generateoutput signals indicative of the light so-detected. The output signalsare processed with the output signals generated by the light from theselected field-of-view of the image being used to generate a measure ofthe gas of interest and the output signals generated by the portion ofthe calibrating light traversing the first and second paths being usedto calibrate the GFCR system.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent upon reference to the following description of thepreferred embodiments and to the drawings, wherein correspondingreference characters indicate corresponding parts throughout the severalviews of the drawings and wherein:

FIG. 1 is a schematic view of a conventional gas filter correlationradiometry (GFCR) system employing two single-element detectors andback-end electronics used to balance the system;

FIG. 2 is a schematic view of a GFCR system that produces a defocusedbeam and detects same using two-dimensional arrays with the signaloutputs from each array being summed and then processed to provide ameasure of a target gas in accordance with the present invention;

FIG. 3 is a schematic view of the GFCR system in FIG. 2 further equippedwith an internally-mounted light producer that can be used tocontinuously calibrate the GFCR system;

FIG. 4 is a schematic view of the front end of the GFCR system in FIG. 3further illustrating an embodiment of the light producer;

FIG. 5 is a schematic view of the conventional GFCR system employing twosingle-element detectors that is further equipped with aninternally-mounted light producer that can be used to continuouslycalibrate the GFCR system; and

FIG. 6 is a schematic view of the GFCR system in FIG. 2 further equippedwith partitioning elements in order to generate an image of the scenebeing observed by the GFCR system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a gas filter correlation radiometry (GFCR)system utilizing a two-detector methodology to simultaneously providelight-absorbing and non-light absorbing measurements without requiringany back-end electronics to balance the system. The system structurealso provides for novel approaches to on-board balance calibration anddata analysis calibration.

Referring again to the drawings and more particularly to FIG. 2, a GFCRsystem in accordance with an embodiment of the present invention isshown and is contained within the dashed-line box referenced by numeral100. In GFCR system 100, optical signal transmission between elementsthereof is indicated by dashed-lines and electrical signal transmissionbetween elements thereof is indicated by solid lines that terminate inan arrowhead.

GFCR system 100 provides a measurement of a gas of interest or “targetgas” by processing an external scene light 200 that defines a scenebeing imaged some distance away from GFCR system 100. At the optical“front end” of GFCR system 100, focusing optics 102 define an entranceaperture 104 of GFCR system 100. Focusing optics 102 operate to focuslight 200 impinging on entrance aperture 104 onto a plane where a fieldstop 106 is positioned. That is, an image defined by scene light 200received at entrance aperture 104 is focused at field stop 106. Theimage so-focused is referenced by numeral 150. The particular choice andarrangement of elements used to construct focusing optics 102 can beachieved by a variety of embodiments. Accordingly, it is to beunderstood that the particular choice of elements comprising focusingoptics 102 is not a limitation of the present invention.

Field stop 106 defines a selected field-of-view of focused image 150with the selected image 152 being transmitted to confinement optics 108.As would be understood in the art, confinement optics 108 is any one ormore optical elements that keep light (from selected image 152) fromdiverging outside the optical boundaries of GFCR system 100. In manyinstances, confinement optics 108 is realized by one or more opticalelements that “collimate” or nearly collimate the light of interest(i.e., the light associated with image 152) in order to efficientlytransfer the light along an intended optical path. Accordingly, theoutput of confinement optics 108 typically produces collimated light asindicated by dashed lines 154. However, it is to be understood thatthere may be implementations of the present invention that are optically“short” such that beam divergence is minimal in which case collimatingoptics may not be required.

A spectral filter 110 disposed in the path of collimated light 154passes a specific spectral band of collimated light 154 that isreferenced by dashed line 156. The spectral band will include thewavelength(s) that would be absorbed in the presence of the target gasof interest.

The spectral band passed by spectral filter 110 impinges on beamsplitting optics 112 (e.g., beam splitter, mirror, etc.) where light 156is split and transmitted as two spatially-separated and nearly identicallight beams 156A and 156B for further processing. Light 156A travelsalong a path that will not cause absorption (or significant attenuationor spectral interference) of any wavelength(s) of light that would beabsorbed by the target gas of interest. To exhibit such “non-absorbing”characteristics, the path along which light 156A travels can be definedby a media or region that achieves this result. Accordingly, the regionalong which light 156A travels could be evacuated, filled with a gaseousmedia that is non-absorbing, or contain a solid material that istransparent or nearly transparent to the spectral band of light 156A andnon-absorbing with respect to wavelength(s) of light that can beabsorbed by the target gas of interest. The method/structure used toachieve the non-absorbing characteristics of the path traveled by light156A is not a limitation of the present invention.

Disposed in the path of light 156A are a diffuser 114 and atwo-dimensional array of optical (e.g., light) detection elements or“two-dimensional detector array” 116 as it will be referred to herein.Detector array 116 is any high-density, two-dimensional array ofelements where each element can detect light and generate an electricalsignal that corresponds to the light's intensity. Such detector arraysare available commercially from a variety of vendors.

The choice of detector array typically depends on many factors toinclude, for example, spectral response, sensitivity, and other specificapplication details that are not pertinent to or limitations on thepresent invention. Also, the particular size of the array (i.e., numberof pixels), the array's frame (read-out) rate, and other specificationsare not limitations of the present invention.

Diffuser 114, which is positioned optically ahead of detector array 116,represents optical elements that cooperate with preceding opticalelements in GFCR system 100 to diffuse or defocus image 152. Morespecifically, each portion or point of image 152 at the field-of-view(FOV) of field stop 106 is identically distributed (as referenced bydashed line 158) as it impinges across some or all of the lightdetection elements of detector array 116. That is, diffuser 114represents any optical element(s) needed to assure that each portion orpoint of light in image 152 at field stop 106 is spread or smeared overdetector array 116 in the same proportion as any other point of light inthe same image. In this way, detector array 116 senses a change in lightfrom the scene (i.e., a change in external scene light 200) witheffectively a constant ensemble response regardless of which portion ofthe scene changed.

By way of a non-limiting example, diffuser 114 can assure this identicaldistribution (or defocus) of image 152 (at the FOV of field stop 106) bycreating an image of entrance aperture 104 on the image plane ofdetector array 116 that is defined by the light detection elementsthereof. This can be explained as follows. Light emanating from a pointat field stop 106 that is traced back through focusing optics 102 willinduce a uniform or even distribution of light intensity across entranceaperture 104. This is also true for the sum of all points in the planeof field stop 106. Therefore, by taking the image emanating from fieldstop 106 (i.e., image 152) and using it to create an image of entranceaperture 104 at the image plane of detector array 116, light from eachpoint at field stop 106 will be more uniformly or evenly distributedacross the image plane of detector array 116. In effect, this creates atotally defocused far-field image. Thus, a change in light intensityemanating from any point in the FOV of field stop 106 will induce achange that is identical to a similar intensity change experienced byany other point in the FOV. This nearly eliminates the problem of falsedifference signals between detectors caused by non-uniform detectorresponse for the detector elements (or non-uniform detector surfaceresponse for single-element detectors).

Disposed in the path of the light 156B are a gas cell 122, a diffuser124, and a two-dimensional array of optical (i.e., light) detectionelements referred to as two-dimensional detector array 126 that issimilar to detector array 116. While detector arrays 116 and 126 areshown as physically distinct elements, the present invention is not solimited. That is, the function of detector arrays 116 and 126 could alsobe realized using a single two-dimensional detector array havingindependently accessible and addressable detector areas that couldsimultaneously and independently receive/process two distinct beams oflight.

Gas cell 122 performs the same function as gas cell 30 (FIG. 1) used inconventional GFCR systems. That is, gas cell 30 is filled with thetarget gas that will absorb light at wavelength(s) passed by spectralfilter 110. The resulting light passed by gas cell 122 is transmitted todiffuser 124 that functions in the same way as diffuser 114.Accordingly, diffuser 124 cooperates with the preceding optical elementsin GFCR system 100 to diffuse or defocus image 152 in a way that causesit to be evenly distributed as it impinges across some or all of thelight detection elements of detector array 126. Similar to diffuser 114,diffuser 124 can assure this even distribution of the image by creatingan image of entrance aperture 104 on the image plane of detector array126 as described above for diffuser 114/detector array 116.

The individual light detection element outputs from each of detectorarrays 116 and 126 are electrical signal representations of the detectedlight intensity. These electrical signals are processed directly togenerate a measure of the target gas of interest present in the scenerepresented by light 200. Processing of these electrical signal outputscan be achieved by a variety of hardware configurations withoutdeparting from the scope of the present invention. For example, theprocessing can be achieved by a single processor or multiple processors.Accordingly, it is to be understood that the separate functional blocksused to represent the processing structure in FIG. 2 are used simply tofacilitate a description of the present invention.

The outputs from detector array 116 are summed at a summer 130 togenerate a signal S₁, and the outputs from detector array 126 are summedat a summer 132 to generate a signal S₂. The sum signals S₁ and S₂ areused by a processor 134 to generate a measure of the target gas ofinterest present in the scene represented by light 200. In general,processor 134 calculates a normalized difference using signals S₁ andS2. More specifically, processor 134 generates a difference between S₁and S₂, and normalizes (i.e., divides) this difference using S₁, S2, orthe sum of S₁ and S₂. The normalized difference is directly dependent onthe light absorption difference between the two signals, but isindependent of the light intensity.

The advantages of the present invention are numerous. The presentinvention creates a totally defocused image, detects the defocused imageusing a two-dimensional detector array, and requires only simple signalsummation and subsequent difference signal processing for a two-detectorGFCR method. This GFCR system eliminates the problems associated withthe conventional two-detector GFCR system that requires sensitiveback-end difference signal balance and gain electronics. As a result,the GFCR system of the present invention provides a novel and robustapproach to GFCR.

By detecting a defocused beam using tens to thousands of rapid read-outpixels from high-density detector arrays, the sum of the pixel readingsgenerated in the present invention serves as a very high-precision anddirect measure of the beam intensity. For example, the HAWAII detectorarray series by Teledyne Imaging Sensors, with array sizes of 1000×1000(i.e., 10⁶ pixels), can be read at KHz (10³/sec) frame rates with welldepths of 10⁵ electrons, and per-pixel read-out noise under 100electrons. This allows counts of up to 10⁶×10³×10⁵=10¹⁴ electrons in onesecond of integration time, yielding potential signal-to-noise limits ofthe square root of the counts or 10⁷. A typical approach would sum pixeloutputs by right shifting digital values as needed during the summationprocess to limit the final sum to the desired digital word size (e.g.,32 bits allows integers of 4.3×10⁹).

The present invention also improves detection linearity since lightbeing measured can be spread over a relatively large surface area toreduce irradiance (flux power/area). Surface response uniformity of thedetector array is far superior to single-element detectors to therebyfurther enhance the defocusing effect. By using detector arrays, theeffect can be enhanced further by well know “flat-fielding” proceduresthat mathematically correct the output to the equivalent of a detectorarray system with perfectly uniform response. This nearly eliminatesfalse difference signals due to scene irregularity that can correlatewith detector surface response irregularity. In summary, the GFCR'sdefocusing aspects are fully exploited by the present invention's noveluse of two-dimensional detector arrays.

The present invention also exhibits the advantageous measurementcharacteristics of spatial and temporal simultaneity where exactly thesame scene is measured at exactly the same time. This eliminates thesystemic scene variation error inherent in modulated single-detectorsystems.

The above-described GFCR system can be further enhanced by providing anon-board calibration system. Referring now to FIG. 3, a GFCR system 300includes all of the elements of GFCR system 100 which are referencedusing the same reference numerals. In addition, GFCR system 300 has aninternally-mounted calibration light producer. In general, a portion ofthe internally-generated light will traverse the same path as externalscene light 200. That is, the portion of the internally-generated lightthat travels the same path as the scene light is confined/collimated,spectrally filtered, split, and ultimately distributed over the imagingplane of detector arrays 116 and 126 similar to the way that externalscene light 200 is processed and distributed over detector arrays 116and 126. In addition, the difference in distribution can be calibratedand mathematically corrected during data processing to produce a nearlyperfect distributional match and superior calibration signal. Thiscorrection process includes ignoring the output signals generated by“pixels” that do not receive scene light. In effect, this allows thecalibration procedure to reject any part of the calibrating light thatdoes not traverse the same path as the scene light.

The purpose of the calibration light producer is to create light thatappears to emanate from within the scene image at field stop 106. Thiscould be done in a variety of ways. For example, light emitting elements(not shown) could be placed within the aperture of field stop 106 or, asdepicted by way of example in FIGS. 3 an 4, a light producer 140 couldbe used that illuminates scattering elements (not shown in FIG. 3 tomaintain clarity in the illustration) placed within the aperture offield stop 106. Since the light generated by light producer 140 isgenerally dispersed over a wide angle relative to the scene lightpassing through field stop 106, only a portion of the calibrating lightfrom light producer 140 will traverse the same path as the scene light.The portion of the calibration light that traverses the same path asexternal scene light 200 falls on the detector arrays in a nearlyidentical distribution over the detector elements (pixels) used to imageGFCR system aperture 104. As described above, the analysis discardssignals from other pixels.

The signal from the calibration light can be distinguished from thescene light by blocking the scene light when the calibration light ison, or simply measuring the difference in signals between states oflight-on and light-off. The calibration signal becomes a measure of afalse difference signal produced by the drift in system response. Thisis equivalent to the signal that would be observed by looking at anexternal unattenuated light source. In this way, GFCR system 300includes the means to accurately and precisely estimate the change indetector array response with the confidence that each detector array isbeing used for calibration in exactly the same proportion as would bethe case for external scene light 200. The use of detector arrays makesit mathematically possible to correct for residual differences inillumination uniformity of the scene light vs. the calibration light,which is a major improvement in calibration capability.

For continuous calibration capability, it typically will be desirable toturn light producer 140 on and off in accordance with a known sequence(e.g., a periodic blinking sequence). Accordingly, an on/off controller142 can be coupled to light producer 140. The “on” portion of the,sequence will generate light that is used to create summed calibrationsignals from summers 130 and 132, with the summed calibration signalsthen being used to generate a normalized difference as was done forlight 200.

When GFCR system 300 is in the calibration mode (i.e., light producer140 is on), external scene light 200 can continue to be present or canbe blocked. In either case, it is the change in output signal (whenlight producer 140 is on vs. when it is off) that is used to calibratethe system. For the best measurement precision, it is preferable toblock light 200 during the time that light producer 140 is on so that aconstant offset is attained. This means that GFCR system 300 must beequipped with some means (not shown) to block light 200 when in thecalibration mode. If such light blocking means is not available orpractical, calibration can still be achieved in the presence of bothlight 200 and light from light producer 140. In such a case, calibrationmust be performed when light 200 is not “noisy” or by statisticalprocessing to determine the change in the output signal when lightproducer 140 is on.

A variety of embodiments of light producer 140 can be employed withoutdeparting from the scope of the present invention. For example, asillustrated in FIG. 4, light producer 140 can be achieved by mounting agrid element 140A in the aperture of field stop 106, and providing alight source 140B at a position that is optically behind grid element140A with respect to entrance aperture 104. When light source 140B isturned on by controller 142, the light therefrom is scattered by gridelement 140A with the scattered light then being defused across theimage plane of the detector array as previously described.

Optimum use of the calibration signal requires that the calibration beamdistribution on the detector array be well matched to the scene beamdistribution on the detector array. Such matching can be achievedoptically or can be achieved mathematically during data processingprovided individual “pixel” values of the detector array can bemeasured/read.

The advantages of using light producer 140 are not limited to the usethereof in the present invention's novel GFCR system. That is, lightproducer 140 and controller 142 could be incorporated into theconventional GFCR system (shown in FIG. 1) utilizing single-elementdetectors and back-end balancing electronics. Accordingly, FIG. 5illustrates the incorporation of light producer 140 and controller 142in a conventional GFCR system (e.g., GFCR system 10 shown in FIG. 1).

Each of GFCR systems 100 and 300 provide an improved target gasdetection system suitable for use in many applications where the actualimage of the area being evaluated is of little or no interest. Indeed,since defocusing of the image is essential to achieve the elimination offalse difference signals due to scene intensity variation, the actualimage is not discernable at the image plane of detector arrays 116 and126. Therefore, for applications requiring the image to be resolved, theGFCR system of the present invention can be further equipped such thatthe defocus advantage is maintained in a structure that also providesfor the reconstruction of the image at the detector arrays.

The re-imaging features to be described below can be incorporated intoeither of GFCR systems 100 or 300 without departing from the scope ofthe present invention. By way of an illustrative example, the re-imagingfeatures of the present invention are illustrated in a GFCR system 400in FIG. 6 where system 400 is an enhancement of the previously-describedGFCR system 100 (FIG. 2).

In general, GFCR system 400 partitions the field-of-view (FOV) at thefocal plane of focusing optics 102 into multiple FOV “pieces” which arere-assembled in the same fashion at the image plane of detector arrays116 and 126. More specifically, a FOV partitioning element 402 ispositioned at the focal plane of focusing optics 102. Partitioningelement 402 comprises a field stop having an array of apertures 404formed therethrough that effectively partition the image impingingthereon into a corresponding array of FOV “pieces”. The size, numberand/or shape of apertures 404 (and the resulting FOV pieces) can betailored for a specific application and, therefore, are not limitationson the present invention. If the GFCR system is to include theabove-described calibration features, partitioning element 402 can bebacked with a light-scattering grid element (not shown) with a lightsource (not shown) being provided at a position that is optically behindthe grid element as previously described.

Each resulting FOV “piece” passed by partitioning element 402 iscollimated and split into two spatially-separated paths as previouslydescribed. In each path, focusing optics are used to re-image the FOVpieces onto another partitioning element that matches partitioningelement 402 as will be explained further below. Specifically, in thenon-absorbing path, focusing optics 406 is disposed to focus the FOVpieces onto a partitioning element 408 having apertures 410 formedtherethrough that are matched to apertures 404. A diffuser 412 ispositioned in each of apertures 410. Partitioning element 408 ispositioned right at the image plane of detector array 116. In a similarfashion, focusing optics 414 are disposed to focus the FOV pieces onto apartitioning element 416 having apertures 418 formed therethrough thatare matched to apertures 404. A diffuser 420 is positioned in each ofapertures 418. Partitioning element 416 is positioned right at the imageplane of detector array 126.

In operation, rather than defocusing the image onto each detector array,GFCR system 400 re-images the FOV from the collimated light at apartitioning diffuser (i.e., the partitioning and diffusing elements ineach path) that “matches” the FOV pieces generated at partitioningelement 402. The term “matches” as used herein means that each FOV piececreated at partition 402 will pass through a corresponding apertureformed in one of partitioning elements 408 or 416. This insures thatlight from different FOV pieces will not mix with light from other FOVpieces so that light from each FOV piece impinges on an independent setof light detecting elements of the corresponding detector array.

Scene resolution will be based on the effective resolution of thepartitioning elements. The outputs of detector array 116 and 126 can beprocessed in subsets of pixels corresponding to each area/zone definedby the partitioning elements. For example, each subset of pixels couldbe summed, differenced, and normalized independently. Thus, thisembodiment of the present invention achieves all of the defocusadvantages described above while still generating an image of the sceneof interest.

Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in light of theabove teachings. For example, the GFCR system of the present inventioncould use additional optical elements (e.g., polarizers) that may berequired for certain applications, but do not impact the functions ofthe present invention. Such elements can be included without departingfrom the scope of the present invention. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced other than as specifically described.

1. A gas filter correlation radiometry (GFCR) system, comprising: anoptical train having an entrance aperture, said optical train adapted toreceive light from a scene of interest at said entrance aperture, saidoptical train (i) using the light to form an image of the scene at afocal plane within said optical train, (ii) confining light from aselected field-of-view of the image at said focal plane to a spectralband at which a gas of interest absorbs, and (iii) splitting theconfined light into first and second paths; light-producing means forselectively producing a calibrating light from within said optical trainat said focal plane wherein a portion of said calibrating lighttraverses each of said first and second paths; a region that issubstantially non-interfering with respect to said spectral band, saidregion being disposed in said first path; a gas cell filled with the gasof interest, said gas cell disposed in said second path; detecting meansfor independently detecting light passed through said region and passedthrough said gas cell and for generating output signals indicative ofsaid light so-detected; and processing means coupled to said detectingmeans for processing said output signals wherein said output signalsresulting from the light from said selected field-of-view of the imageare used to generate a measure of the gas of interest and said outputsignals resulting from said portion of said calibrating light are usedto calibrate said GFCR system.
 2. A GFCR system as in claim 1 whereinsaid region is evacuated.
 3. A GFCR system as in claim 1 wherein saidregion contains a gaseous media.
 4. A GFCR system as in claim 1 whereinsaid region contains a solid media.
 5. A GFCR system as in claim 1wherein said light-producing means periodically turns said calibratinglight on and off.
 6. A GFCR system as in claim 1 wherein said detectingmeans comprises: a first optical detector defined by a two-dimensionalarray of optical detection elements disposed in said region and alongsaid first path; a first diffuser disposed in said region at a positionthat is optically in front of said first optical detector, said firstdiffuser assuring that each portion of the light from said selectedfield-of-view of the image is identically distributed across at least aportion of said optical detection elements of said first opticaldetector; a second optical detector defined by a two-dimensional arrayof optical detection elements and disposed along said second path; and asecond diffuser disposed at a position that is optically between saidgas cell and said second optical detector, said second diffuser assuringthat each portion of the light from said selected field-of-view of theimage is identically distributed across at least a portion of saidoptical detection elements of said second optical detector.
 7. A GFCRsystem as in claim 6 wherein said first diffuser images said entranceaperture at a plane aligned with said optical detection elements of saidfirst optical detector.
 8. A GFCR system as in claim 6 wherein saidsecond diffuser images said entrance aperture at a plane aligned withsaid optical detection elements of said second optical detector.
 9. Agas filter correlation radiometry (GFCR) system, comprising: focusingoptics having an entrance aperture through which external light from ascene of interest passes, said focusing optics focusing the light onto afocal plane to form an image; a field stop positioned at said focalplane for permitting light from a selected field-of-view of the imageso-focused to pass therethrough; light-producing means positionedoptically behind said field stop for selectively producing a calibratinglight at said focal plane; collimating optics for collimating at leastone of the light from said selected field-of-view of the image and aportion of said calibrating light to generate collimated light; aspectral filter disposed to receive said collimated light and pass aspectral band thereof at which a gas of interest absorbs; beam splittingoptics disposed to receive said spectral band of said collimated lightso-passed and split same into first and second paths; a region that issubstantially non-interfering with respect to said spectral band, saidregion being disposed in said first path; a first optical detectordefined by a two-dimensional array of optical detection elementsdisposed in said region; a first diffuser disposed in said region suchthat said first beam impinges thereon at a position that is optically infront of said first optical detector, said first diffuser assuring thateach portion of the light from said selected field-of-view of the imageand said portion of said calibrating light is identically distributedacross at least a portion of said optical detection elements of saidfirst optical detector wherein each of said optical detection elementsof said first optical detector generates an output signal; a gas cellfilled with the gas of interest, said gas cell disposed along saidsecond path; a second optical detector defined by a two-dimensionalarray of optical detection elements; a second diffuser disposed at aposition that is optically between said gas cell and said second opticaldetector, said second diffuser assuring that each portion of the lightfrom said selected field-of-view of the image and said portion of saidcalibrating light is identically distributed across at least a portionof said optical detection elements of said second optical detectorwherein each of said optical detection elements of said second opticaldetector generates an output signal; and processing means coupled tosaid first and second optical detectors for (i) summing said outputsignals generated by said first optical detector to form a first sum,(ii) summing said output signals generated by said second opticaldetector to form a second sum, and (iii) using said first and secondsums to generate a measure of the gas of interest and to calibrate saidGFCR system.
 10. A GFCR system as in claim 9 wherein said region isevacuated.
 11. A GFCR system as in claim 9 wherein said region containsa gaseous media.
 12. A GFCR system as in claim 9 wherein said regioncontains a solid media.
 13. A GFCR system as in claim 9 wherein saidfirst diffuser images said entrance aperture at a plane aligned withsaid optical detection elements of said first optical detector.
 14. AGFCR system as in claim 9 wherein said second diffuser images saidentrance aperture at a plane aligned with said optical detectionelements of said second optical detector.
 15. A GFCR system as in claim9 wherein said processing means generates a difference between saidfirst and second sums and normalizes said difference using at least oneof said first and second sums.
 16. A GFCR system as in claim 9 whereinsaid light-producing means periodically turns said calibrating light onand off.